Photovoltaic panels contain a large number of series connected photovoltaic cells. In optimal conditions, the cells of a panel may be equally irradiated and crossed by the current delivered to an external circuit and eventually to an electric load. However there may be cases in which some cells of a panel are partially or completely shadowed (by leaves or other opaque objects, clouds, projected shadows, etc.). Even if only temporarily, sub-illuminated cells limit the flow of current generated by other cells in series thereto, that may be fully illuminated, thus significantly reducing the current yielded by the panel.
In case of almost complete darkening, darkened cells behave as inversely biased diodes that are a load for the other illuminated cells. Moreover, the current generated by the functioning cells may cause an overvoltage on the darkened cells that may surpass the breakdown voltage of the darkened cell.
This phenomenon, known with the name “hot-spot”, may overheat the darkened cells and in certain cases even impair them irreparably with a consequent decline of current yield. Substantially, sub-illuminated and/or darkened cells dissipate electric power generated by the other cells.
To prevent damaging cells irreparably, in the photovoltaic panel and more precisely inside the so-called junction box of the panel, diodes having a low voltage drop in conduction (usually Schottky diodes) define “by-pass” diodes electrically connected or coupled in anti-parallel to segments of strings of series connected cells. These provide local low-resistance shunt current paths, thus by-passing a respective segment that contains the darkened cells of the string.
This approach protects the cells and the panels from permanent impairment, and may prevent a whole string of series connected cells from being permanently damaged excessively limited in ability to produce useful current. In practice, the shunt current is transferred to the by-pass diode that tends to overheat when shunting the current generated by the numerous fully illuminated cells. To minimize power dissipation, common Schottky diodes are used, nevertheless efficiency is noticeably penalized. A measure of such an energy conduction loss may be given by conventional connected panels realized with 8″ cells, wherein the generated current may reach up to about 15 A, with a consequent potential power dissipation of several tens of Watts.
To obviate this, the document DE-102005036153-A1 discloses the use of a power MOS transistor as a by-pass diode. During normal functioning of the cells of the panel, the transistor remains off. If a certain string of cells or some of these are darkened, the current generated by the irradiated cells inverts the voltage on the by-pass MOS transistor by flowing in the intrinsic diode of the integrated structure of the transistor.
The voltage drop on the intrinsic diode is used, through an inductive circuit, for loading a capacitor. After a time defined by the time constant of the circuit, the MOS is turned on through a dedicated driving circuit, thus reducing the voltage drop down to few tens of mV, and thus reducing the dissipated power down to few hundreds of mW. When a second time constant elapses, the transistor is turned off, thus recharging the capacitor during the successive phase and so on, according to a cyclical functioning mode.